Methods and apparatus for frequency offset estimation

ABSTRACT

Methods and apparatus for frequency offset estimation are disclosed. In an exemplary embodiment, a method includes determining a demodulation reference signal (DMRS) frequency offset estimate from DMRS symbols in a received signal, and determining a cyclic prefix (CP) frequency offset estimate from cyclic prefix values in the received signal. The method also includes combining the DMRS and CP frequency offset estimates to determine a final frequency offset estimate. In an exemplary embodiment, an apparatus includes a DMRS frequency offset estimator that determines a DMRS frequency offset estimate based on DMRS symbols received in an uplink transmission, and a cyclic prefix (CP) frequency offset estimator that determines a CP frequency offset estimate based on cyclic prefix values in the uplink transmission. The apparatus also includes an offset combiner that combines the DMRS frequency offset estimate with the CP frequency offset estimate to generate a final frequency offset estimate.

CLAIM TO PRIORITY

This application claims the benefit of priority based upon U.S.Provisional Patent Application having Application No. 62/335,366, filedon May 12, 2016, and entitled “FREQUENCY OFFSET ESTIMATOR,” which ishereby incorporated herein by reference in its entirety.

FIELD

The exemplary embodiments of the present invention relate totelecommunications networks. More specifically, the exemplaryembodiments of the present invention relate to receiving and processingdata streams via a wireless communication network.

BACKGROUND

With a rapidly growing trend of mobile and remote data access overhigh-speed communication networks, such as provided by long termevolution (LTE) cellular networks, accurate delivery and deciphering ofdata streams has become increasingly challenging and difficult. Forexample, in a multi-user LTE system, the frequency offset associatedeach user is independent of the other users. Frequency offset causesclockwise or counter clockwise incremental phase rotation of thereceived signal samples. The current LTE standard constrains thefrequency offset that can be estimated to a range of (−1 kHz to 1 kHz).However, performance requirements specified in the LTE specificationforces baseband system manufacturers to extend the frequency offsetestimation range. For example, to provide communications to users ridingon high speed trains, expanding the frequency offset estimation to arange that covers (−1.7 kHz to 1.7 kHz) may be necessary.

Therefore, it is desirable to have a mechanism for frequency offsetestimation that can accurately estimate a wider range of frequencyoffset to satisfy high speed performance requirements of wirelesscommunication systems.

SUMMARY

The following summary illustrates simplified versions of one or moreaspects of present invention. The purpose of this summary is to presentsome concepts in a simplified description as more detailed descriptionthat will be presented later.

Methods and apparatus for frequency offset estimation are disclosed. Forexample, the frequency offset estimation for use in an LTE uplink can beperformed. The exemplary embodiments of the frequency offset estimationsupport frequency offset estimation with 15 times wider range than theconventional schemes. The exemplary embodiments of the frequency offsetestimation utilize joint frequency offset estimation that comprises bothfine and coarse frequency offset estimation. The exemplary embodimentsof the frequency offset estimation can be implemented with a singlepipeline and the frequency offset can be estimated up to subcarrierlevel (smallest element of the FFT output).

The exemplary embodiments of the frequency offset estimation can use theuser allocation information in layer 2 (MAC layer), such that theestimation accuracy can be improved through the average of the estimateof each sub-frame.

The exemplary embodiments of the frequency offset estimation can beapplied to LTE uplink MIMO cases where the cyclic prefix is notcompletely orthogonal to the other part of a symbol by including thenormal data part to keep the orthogonality.

In an exemplary embodiment, a method is provided that includesdetermining a demodulation reference signal (DMRS) frequency offsetestimate from DMRS symbols in a received signal, and determining acyclic prefix (CP) frequency offset estimate from cyclic prefix valuesin the received signal. The method also includes combining the DMRS andCP frequency offset estimates to determine a final frequency offsetestimate.

In an exemplary embodiment, an apparatus is provided that includes aDMRS frequency offset estimator that determines a DMRS frequency offsetestimate based on DMRS symbols received in an uplink transmission, and acyclic prefix (CP) frequency offset estimator that determines a CPfrequency offset estimate based on cyclic prefix values in the uplinktransmission, The apparatus also includes an offset combiner thatcombines the DMRS frequency offset estimate with the CP frequency offsetestimate to generate a final frequency offset estimate.

Additional features and benefits of the exemplary embodiments) of thepresent invention will become apparent from the detailed description,figures and claims set forth below.

BRIEF DESCRIPTION OF THE DRAWINGS

The exemplary aspects of the present invention will be understood morefully from the detailed description given below and from theaccompanying drawings of various embodiments of the invention, which,however, should not be taken to limit the invention to the specificembodiments, but are for explanation and understanding only.

FIG. 1 shows a communication network comprising a transceiver having anexemplary embodiment of a frequency offset estimator (OE) configured toaccurately measure frequency offset associated with uplinkcommunications from a plurality of users;

FIG. 2 shows an exemplary functional block diagram of the communicationnetwork shown in FIG. 1;

FIG. 3 shows an exemplary diagram of data symbols received at a receiverin uplink transmissions from a plurality of users;

FIG. 4 shows an exemplary embodiment of a DMRS frequency offsetestimator for use in an exemplary embodiment of the frequency offsetestimator shown in FIGS. 1-2;

FIG. 5 shows an exemplary embodiment of a user data stream that includesdata values having a cyclic prefix;

FIG. 6 shows an exemplary embodiment of a frequency offset estimatorconstructed in accordance with the present invention; and

FIG. 7 shows an exemplary method for performing frequency offsetestimation in a receiver in accordance the present invention.

DETAILED DESCRIPTION

Aspects of the present invention are described herein the context ofmethods and apparatus for frequency offset estimation.

The purpose of the following detailed description is to provide anunderstanding of one or more embodiments of the present invention. Thoseof ordinary skills in the art will realize that the following detaileddescription is illustrative only and is not intended to be in any waylimiting. Other embodiments will readily suggest themselves to suchskilled persons having the benefit of this disclosure and/ordescription.

In the interest of clarity, not all of the routine features of theimplementations described herein are shown and described. It will, ofcourse, be understood that in the development of any such actualimplementation, numerous implementation-specific decisions may be madein order to achieve the developer's specific goals, such as compliancewith application- and business-related constraints, and that thesespecific goals will vary from one implementation to another and from onedeveloper to another. Moreover, it will be understood that such adevelopment effort might be complex and time-consuming, but wouldnevertheless be a routine undertaking of engineering for those ofordinary skills in the art having the benefit of embodiments of thisdisclosure.

Various embodiments of the present invention illustrated in the drawingsmay not be drawn to scale. Rather, the dimensions of the variousfeatures may be expanded or reduced for clarity. In addition, some ofthe drawings may be simplified for clarity. Thus, the drawings may notdepict all of the components of a given apparatus (e.g., device) ormethod. The same reference indicators will be used throughout thedrawings and the following detailed description to refer to the same orlike parts.

In various exemplary embodiments, the disclosed methods and apparatusprovide one or more of the following novel aspects.

A. Frequency offset estimation using the cyclic prefix.

B. Frequency offset estimation in frequency domain by measuring thephase difference of two FFT outputs generated with two input samplescaptured at normal and adjustable time offset.

C. Phase difference measurement between a normal FFT output of an LTEuplink symbol and an FFT output of a cyclic-time shifted. LTE uplinksymbol, which is captured with an adjustable sample offset, whichinjects timing offset.

D. The complex metrics for phase difference estimation arecross-correlation results (a product of complex values of eachsubcarrier), and they are averaged for each resource block (RB) as abasic unit. The unit (RB-wise) metrics are combined across the RBs foreach user depending on uplink scheduling, and across several time slots(TTI) for better accuracy.

FIG. 1 shows a communication network 100 comprising a transceiver 116having an exemplary embodiment of a frequency offset estimator (OE) 118configured to accurately measure frequency offset associated with uplinkcommunications from a plurality of users. The communication network 100includes a base station 114 that includes the transceiver 116. Thetransceiver 116 has a transmitter portion 128 and a receiver portion130. The base station 114 communicates with radio towers 110 located incell site 102.

User equipment (UE) 104 transmits uplink communications 120 to the basestation 114 through tower 110 c, and user equipment 106 transmits uplinkcommunications 122 to the base station 114 through tower 110 b. Forexample, the UEs can be cellular phones, handheld devices, tabletcomputers or iPad® devices. It should be noted that the underlyingconcepts of the exemplary embodiments of the present invention would notchange if one or more devices were added or removed from thecommunication network 100.

Each user equipment transmits its signal through an associated uplinkchannel. The transmitted uplink signal suffers from frequency offset dueto Doppler shift and mismatch between oscillators in the transmitter ofthe UE and the receiver in the base station. Each user's frequencyoffset can be assumed to be independent from other users, since eachuser has its own oscillator and its motion and speed are independentfrom other users.

To improve data reception for the uplink communications, the receiver130 includes the frequency offset estimator 118. The frequency offsetestimator 118 operates to estimate the frequency offset associated witheach of the users 104 and 106. In an exemplary embodiment, the OE 118estimates the frequency offset with a wide range (e.g., −1.7 kHz to 1.7kHz), which enables accurate data reception on the uplink communicationseven when the users 104 and 106 are traveling at a high rate of speed.For example, a user may be riding on a high-speed train traveling atspeeds approaching or exceeding 300 mph and the OE 118 will still beable to determine an accurate frequency offset. A more detaileddescription of the OE 118 is provided below.

FIG. 2 shows an exemplary functional block diagram 200 of thecommunication network 100 shown in FIG. 1. Each user equipment (202)transmits its signal through an uplink transmission to the receiver 130.The transmitted signals suffer from associated frequency offset (204)due to Doppler shift and mismatches between oscillators in the userequipment (transmitters) and oscillators in the receiver 130 (basestation). Each user's frequency offset can be assumed to be independentsince each user has its own oscillator and is moving at a speed that isindependent of the other users. In various exemplary embodiments, thefrequency offset estimator 118 operates to estimate the frequency offsetof each of the user with a wide range (e.g., −1.7 kHz to 1.7 kHz), whichenables accurate data reception of the uplink transmission even when auser is traveling at a high rate of speed.

FIG. 3 shows an exemplary diagram of data symbols 300 received at areceiver in uplink transmissions from a plurality of users. For example,the symbols 300 may be received by the receiver 130 shown in FIG. 2.With respect to an LTE uplink transmission, each user's signal isconfigured as SC-FDMA (single carrier—frequency division multipleaccess), which is multiplexed in the frequency domain with other users.Each users' uplink transmission includes two DMRS (demodulationreference signal) symbols (302, 304) per a sub-frame, where a sub-frameis the minimum transmission unit in LTE.

The DMRS symbols work as pilot symbols, which enable channel estimationand frequency offset estimation. When they are used for frequency offsetestimation, the phase difference between two channel responses estimatedwith them is measured. If the measured phase shift is ϕ, the frequencyoffset can be calculated as:f _(offset)=(ϕ/2πT)where T is time distance between the two DMRS, which is 0.5 ms in LTEsystems. Thus, the estimation range is limited to (−1 kHz, 1 kHz), sinceit is difficult to distinguish the phase difference beyond the range of(−π to π).

FIG. 4 shows an exemplary embodiment of a DMRS frequency offsetestimator 408 for use in an exemplary embodiment of the frequency offsetestimator 118 shown in FIGS. 1-2. In an exemplary embodiment, theestimator 408 comprises a digital signal processor (DSP). A receivedsignal is input to user separator 402, which performs an (FFT) toseparate out frequencies associated with a particular user. For example,since the LTE uplink uses SC-FDMA (single carrier—frequency domainmultiple access) scheme to multiplex multi-user signals in frequencydivision manner, the receiver 130 operates to measure the frequencyoffset of each user by separating each user signal in the frequencydomain. The output of the separator 402 is input to the DMRS frequencyoffset estimator 408, which comprises a phase difference calculator 404and a time interval divider 406.

In an exemplary embodiment, the phase difference calculator 404comprises at least one of a CPU, processor, state machine, logic,memory, discrete hardware and/or any combination thereof. The phasedifference calculator 404 calculates a phase different between two DMRSsymbols in a received sub-frame for a particular user. For example, asillustrated in FIG. 3, the phase difference calculator 404 calculatesthe phase shift ϕ between DMRS symbols 302 and 304.

In an exemplary embodiment, the time interval divider 406 comprises atleast one of a CPU, processor, state machine, logic, memory, discretehardware and/or any combination thereof. The time interval divider 406divides the calculated phase shift ϕ by the time interval (T) betweenthe DMRS symbols (e.g., symbols 302 and 304) to determine a DMRSfrequency offset estimate. If the measured phase shift is ϕ, the DMRSfrequency offset can be calculated as:f _(offset)=(ϕ/2πT)where T is time distance between the two DMRS, hick, for example, is 0.5milliseconds (ms) in LTE systems.

FIG. 5 shows an exemplary embodiment of a user data stream 500 thatincludes data values having a cyclic prefix. In an exemplary embodiment,the DATA1 symbol has a cyclic prefix (CP1) that is a copy of the data502 at the end of the DATA1 symbol. The data 502 is copied and thenplaced in front to prefix the symbol with a repetition of the data atthe end of the symbol. The cyclic prefix serves two purposes. First, asa guard interval, it eliminates the inter-symbol interference from theprevious symbol. Second, as a repetition of the end of the symbol, itallows a linear convolution of a frequency-selective multipath channelto be modelled as a circular convolution, which in turn may betransformed to the frequency domain using a discrete Fourier transform.This approach allows for simple frequency-domain processing, such aschannel estimation and equalization. Typically, the length of the cyclicprefix must be at least equal to the length of the multipath channel.

In an exemplary embodiment, to widen the frequency offset measurementrange of the OE 118 and to support the frequency offset estimation formultiple users in the LTE uplink, the cyclic prefix is exploited duringthe frequency offset estimation. For example, in an exemplaryembodiment, the contribution of the cyclic prefix to the frequencyoffset estimation is separated in the frequency domain and selectivelyapplied.

FIG. 6 shows an exemplary embodiment of a frequency offset estimator 600constructed in accordance with the present invention. For example, thefrequency offset estimator 600 is suitable for use as the frequencyoffset estimator 118 shown in FIGS. 1-2. The frequency offset estimator600 comprises user separator 602, DMRS frequency offset estimator 408,offset combiner 606, time offset adjuster 608, user separator 610,cyclic time shifter 612, cross correlator 614 and frequency/timeaverager 616.

In an exemplary embodiment, the frequency offset estimator 600 estimatesthe frequency offset of each user by estimating a first frequency offsetusing two received DMRS (pilot) symbols and by estimating a secondfrequency offset using cyclic prefixes. The two frequency offsets arecombined to form a final frequency offset estimation with much widerrange than conventional offset detectors. A first processing path 632 isused to estimate the first frequency offset for a particular user usingDMRS symbols for the frequency offset estimation. A second processingpath 634 is used to estimate the second frequency offset for theparticular user using cyclic prefixes. The offset combiner 606 operatesto combine the first and second frequency offsets to produce a finalfrequency offset estimate 630 for the particular user with a wide rangeof up to (−15 kHz to 15 kHz).

In an exemplary embodiment, the user separator 602 comprises at leastone of a CPU, processor, state machine, logic, memory, discrete hardwareand/or any combination thereof to perform user separation in LTE uplink.For example, since the uplink signals are frequency-domain multiplexedsignals, each user's signal can be separated by its own transmittedfrequency. In an exemplary embodiment, the separator 602 performs an FFTon the received signal 618 to separate frequency components associatedwith a particular user.

In an exemplary embodiment, the DMRS frequency offset estimator 408operates as described with reference to FIG. 4. For example, theestimator 408 measures the phase difference between two DMRS symbols andestimates a fine frequency offset in the range of (−1 kHz to 1 kHz). Theoutput of the estimator 408 is a first frequency offset estimate 622that is input to the offset combiner 606.

In an exemplary embodiment, the time offset adjuster 608 comprises atleast one of a CPU, processor, state machine, logic, memory, discretehardware and/or any combination thereof and time adjustments thereceived symbols 618 to take the cyclic prefix portion of thetransmitted symbols and to provide this cyclic prefix portion as aninput to the user separator 610. For example, in the transmitter (userequipment), the cyclic prefix is copied from the tail of each SC-FDMAsymbol to the front of each symbol, so that the front and tail portionof the symbol are identical. Thus, in the receiver side (base station),both parts can be used for frequency offset estimation for each user.The cyclic prefix portion 636 is input to the user separator 610.

In an exemplary embodiment, the user separator 610 comprises at leastone of a CPU, processor, state machine, logic, memory, discrete hardwareand/or any combination thereof to perform user separation in LTE uplinkcommunications. For example, since the uplink signals arefrequency-domain multiplexed signals, each user's signal can beseparated by its own transmitted frequency. In an exemplary embodiment,the separator 602 performs an FFT on the cyclic prefix input 636 toseparate frequency components 638 of the cyclic prefix associated withthe same user whose signal was separated out by the user separator 602.

In an exemplary embodiment, the cyclic time shifter 612 comprises atleast one of a CPU, processor, state machine, logic, memory, discretehardware and/or any combination thereof to perform cyclic time shiftingof the FFT output (frequency components 638) from the user separator610. To equalize the time alignment of time domain symbols for twooffset detection paths, cyclic time shifting is performed. This can bedone in time (before FFT) or frequency (after FFT). In an exemplaryembodiment, the cyclic time shifting is performed by applying complexsinusoid samples to the FFT output (components 638), which results incyclic time shifting of its time domain sequences (cyclic prefix input636). This block rotates the FFT output 624 to be aligned identicallywith FFT output 620 in the first processing path 632.

In an exemplary embodiment, the cross correlator 614 comprises at leastone of a CPU, processor, state machine, logic, memory, discrete hardwareand/or any combination thereof to cross correlate two inputs (the FFToutput sequences 620 and 624) from the first processing path 632 and thesecond processing path 634, so that the correlated output 626 contains acomplex number which can be accumulated over several symbol durations(which is configurable) and contains a phase difference in its phaseangle. According to the frequency portion of each user, complexmultiplication results of two FFT outputs are summed or averaged for thefrequency offset estimation of multiple users.

In an exemplary embodiment, the frequency/time average 616 comprises atleast one of a CPU, processor, state machine, logic, memory, discretehardware and/or any combination thereof to take an average of each ofthe frequency elements of the output 626 from the correlator 614 oversome time duration, to lower the impact of the receiver noise on thephase difference which is measured through cross correlation.

Therefore, by cross-correlating the two FFT outputs (noncyclic timeshifted and cyclic time shifted) one by one, the phase difference ofeach element of the FFT output can be obtained, since there are twodifferent tail parts when the frequency offset is not zero. During thecross-correlation, the normal data portion can be skipped, since thenormal data portion is identical in both signal paths.

In an exemplary embodiment, the frequency offset combiner 606 comprisesat least one of a CPU, processor, state machine, logic, memory, discretehardware and/or any combination thereof. The combiner 606 operates toreceive the DMRS frequency offset estimate 622 from frequency offsetestimator 408, which represents a fine frequency offset (ϕfine), and theaveraged cross-correlation result 628, which represents a coarsefrequency offset (ϕcoarse) determined by the cyclic prefix and itsoriginal part (tail of a normal symbol). The latter is for estimatingthe integer part of the phase difference, which is a “coarse frequencyoffset estimate.” The combining results in a total phase differenceϕtotal that can be written as:Φ_(total)=Φ_(fine)+Φ_(course)=Φ_(fine)+2n _(coarse)πwhere n_(coarse) is an integer from the coarse estimate of the frequencyoffset using the output from the second processing path 634 and can beobtained from:n _(coarse)=round[(Φ_(course)/(2πT _(symb) /T _(dmrs)))]where T_(symb) a symbol duration (1/15 kHz), and T_(dmrs) is a timedistance between two DMRS symbols (0.5 ms).

In an exemplary embodiment, the first processing path 632 is used forfine frequency offset in the range of (−1 kHz, 1 kHz), and the secondprocessing path 634 is used for coarse frequency offset in the range of(15 kHz, 15 kHz).

Techniques to Determine Coarse Offset

Determining the coarse offset with regards to the use of the normal datapart of the symbol, two schemes are presented. In a first scheme, if thenormal data part of the symbol is included in the cross-correlationperformed by block 614, the follow result is produced:X _(corr0) =A+B ^(jΦcoarse)where A is a real constant obtained from the normal data part, andΦ_(coarse) is a phase difference between the cyclic prefix and tail partof each symbol. Thus, the coarse frequency offset can be estimated asfollows:Φ_(coarse)=angle(X _(corr0) −A)where A can be simply estimated by measuring the power of an SC-FDMAsymbol.

In a second scheme, to eliminate the impact of A, the normal data partcan be skipped when cross-correlating at block 614, which results in thefollowing:X _(corr1) =B ^(jΦcoarse)

The coarse phase difference then can be calculated simply as:Φ_(coarse)=angle (X _(corr1))

It should be noted, however, in some cases in LTE uplink MIMO, some DMRSsymbols are not orthogonal to other parts of the symbol, so in that casethe first technique may yield better performance.

FIG. 7 shows a method 700 for performing frequency offset estimation ina receiver in accordance with an exemplary embodiment of the presentinvention. For example, in an exemplary embodiment, the method 700 issuitable for use by the OE 600 shown in FIG. 6.

At block 702, an uplink signal is received at a receiver from one ormore users. For example, the uplink signal 618 comprises uplinktransmissions from one or more users in an LTE communication network.

At block 704, at the start of the first processing path 632, aparticular user transmissions are separated from the received signal. Inan exemplary embodiment, the user separator 602 performs this operationby performing an (FFT) to separate frequency components transmitted bythe particular user and outputting these components.

At block 706, a DMRS frequency offset estimation is performed on thefrequency components for the selected user. For example, the DMRSfrequency offset estimator 408 operates to receive the user frequencycomponents and perform frequency offset estimation using the DMRSsymbols in the received transmissions. A more detailed description ofthe DMRS frequency offset estimator 408 is provided above with referenceto FIG. 4. In an exemplary embodiment, the time interval divider 406divides the calculated phase shift ϕ by the time interval (T) betweenthe DMRS symbols (e.g., symbols 302 and 304) to determine a DMRSfrequency offset estimate. If the measured phase shift is ϕ, the DMRSfrequency offset can be calculated as:f _(offset)=(ϕ/2πT)where T is time distance between the two DMRS, which, for example, is0.5 milliseconds (ms) in LTE systems. The DMRS frequency offset estimaterepresents a fine frequency offset and is output to block 718.

At block 708, at the start of a second processing path 634, a timeoffset of the received signal is adjusted. In an exemplary embodiment,the time offset adjuster 608 adjusts a time offset of the receivedsignal. The received signal with an adjusted time offset is then output.

At block 710, user transmissions are separated from the time adjustedreceived signal. In an exemplary embodiment, the user separator 610performs this operation by performing an (FFT) to separate frequencycomponents transmitted by the selected user (see block 704) andoutputting these components.

At block 712, a cyclic time shift is performed on the separated timeadjusted user transmissions. Since two input signal samples for the twooffset estimation paths are not time aligned, this time offsetdifference is compensated by cyclic time shifting. This can be donebefore or after the FFT process. In an exemplary embodiment, the cyclictime shifter 612 performs this operation.

At block 714, a cross-correlation is performed on the output of block704 and the output of block 712. For example, in an exemplaryembodiment, the cross-correlator 614 performs a cross correlation on theuser frequency components 620 derived from the received signal and thecyclic shifted frequency components 624. For example, after an FFTconverts the received signal samples (in time domain) to frequencydomain samples in each offset estimation path, the resulting samplesfrom the two paths are cross-correlated (1-to-1 complex multiplication)to compute the phase difference between them.

At block 716, frequency/time averaging is performed. In an exemplaryembodiment, the frequency/time average 616 performs this operation byaveraging together multiple correlation outputs over time. The averagedvalue is output as cyclic prefix (CP) frequency offset estimate 628. Forexample, the cross-correlation results for the entire uplink frequencybands are summed or averaged selectively over the subcarrier range thathas been scheduled for each uplink user. The uplink scheduling is beingdone in the receiver side, and this scheduling information is alreadyknown to the receiver. Those values are also accumulated or averagedover the selected symbols in each user's transmitting duration.

At block 718, an operation of combining is performed to combine the DMRSfrequency offset estimate (fine offset) ith the CP frequency offsetestimate (coarse offset). In an exemplary embodiment, the offsetcombiner 606 performs this operation. The output of the combiningoperation is a final frequency offset estimate with 15 times wider rangethan frequency offset estimates using only the DMRS symbols.

While particular embodiments of the present invention have been shownand described, it will be obvious to those skilled in the art that,based upon the teachings herein, changes and modifications may be madewithout departing from this exemplary embodiments of the presentinvention and its broader aspects. Therefore, the appended claims areintended to encompass within their scope all such changes andmodifications as are within the true spirit and scope of this exemplaryembodiments of the present invention.

What is claimed is:
 1. A method, comprising: determining a demodulationreference signal (DMRS) frequency offset estimate from DMRS symbols in areceived signal; determining a cyclic prefix (CP) frequency offsetestimate from cyclic prefix values associated with symbols received inthe received signal, wherein the determining the CP frequency offsetestimate includes adjusting a time offset of the received signal andseparating a selected user's uplink transmission from the time adjustedreceived signal to generate a separated signal; and combining the DMRSand CP frequency offset estimates to determine a final frequency offsetestimate.
 2. The method of claim 1, wherein the operation of determiningthe DMRS frequency offset estimate comprises separating a selecteduser's uplink transmission associated with the DMRS symbols from thereceived signal.
 3. The method of claim 2, wherein the operation ofdetermining the DMRS frequency offset estimate comprises calculating aphase difference ϕ between two DMRS symbols.
 4. The method of claim 3,wherein the operation of determining the DMRS frequency offset estimatecomprises dividing the phase difference ϕ by a selected time interval Tto determine the DMRS frequency offset estimate.
 5. The method of claim4, wherein the operation of determining the DMRS frequency offsetestimate comprises calculating the DMRS frequency offset estimate froman equation expressed as f_(offset)=(ϕ/2πT).
 6. The method of claim 1,wherein the operation of determining the CP frequency offset estimatecomprises cyclic time shifting the separated signal to generate a cyclictime shifted signal.
 7. The method of claim 6, wherein the operation ofdetermining the CP frequency offset estimate comprises cross-correlatinga selected user's uplink transmission associated with the DMRS symbolsand the cyclic time shifted signal to generate a correlated signal. 8.The method of claim 7, wherein the operation of determining the CPfrequency offset estimate comprises frequency time averaging thecorrelated signal to generate the CP frequency offset estimate.
 9. Themethod of claim 8, wherein the operation of combining comprisescombining the DMRS frequency offset estimate (Φfine) with the CPfrequency offset estimate (Φcoarse) to determine a total phasedifference Φtotal from an equation expressed as: Φtotal=fine+Φcoarse.10. The method of claim 9, further comprising an operation ofdetermining Φcoarse from an equation expressed as (2*ncoarse*π) wherencoarse=round [Φcoarse/(2πTsymb/Tdmrs))], and where Tsymb is a symbolduration and Tdmrs is a time distance between two DMRS symbols.
 11. Themethod of claim 10, further comprising an operation of receiving thereceived signal in a long term evolution (“LTE”) uplink transmission.12. An apparatus, comprising: a demodulation reference signal (DMRS)frequency offset estimator that determines a DMRS frequency offsetestimate based on DMRS symbols received in an uplink transmission,wherein the DMRS frequency offset estimator is configured to calculate aphase difference Φ between two DMRS symbols and divide the phasedifference by a selected time interval T to determine the DMRS frequencyoffset estimate from an equation expressed as f_(offset)=(Φ/2πT); acyclic prefix (CP) frequency offset estimator that determines a CPfrequency offset estimate based on cyclic prefix values associated withsymbols in the uplink transmission, wherein the CP frequency offsetestimator is configured to adjust a time offset of the received signalto generate a time adjusted signal and separates an uplink usertransmission from the time adjusted signal; and an offset combiner thatcombines the DMRS frequency offset estimate with the CP frequency offsetestimate to generate a final frequency offset estimate.
 13. Theapparatus of claim 12, wherein the CP frequency offset estimator cyclictime shifts the uplink user transmission to generated a time shiftedsignal, and cross-correlates the time shifted signal with a userseparated signal to generate a correlated signal.
 14. The apparatus ofclaim 13, wherein the CP frequency offset estimator frequency timeaverages the correlated signal to generate the CP frequency offsetestimate.
 15. The apparatus of claim 14, wherein the offset combinercombines the DMRS frequency offset estimate (Φfine) with the frequencyoffset estimate (Φcoarse) to determine a total phase difference Φtotalfrom an equation expressed as: Φtotal=Φfine+Φcoarse.
 16. An apparatus,comprising: a demodulation reference signal (DMRS) estimator configuredto determine a DMRS frequency offset estimate in accordance with DMRSsymbols in a received signal; a cyclic prefix (CP) estimator coupled tothe DMRS estimator and configured to provide a CP frequency offsetestimate in response to cyclic prefix values associated with symbolsreceived in the received signal, wherein the CP estimator is able toadjust a time offset of the received signal and wherein the CP estimatoris configured to separate a selected user's uplink transmission from thetime adjusted received signal to generate a separated signal; and acircuit coupled to the CP estimator and configured to combine the DMRSfrequency offset estimate with the CP frequency offset estimate toidentify a combined frequency offset estimate.
 17. The apparatus ofclaim 16, wherein the DMRS estimator is configured to separate aselected user's uplink transmission associated with the DMRS symbolsfrom the received signal.
 18. The apparatus of claim 16, wherein theDMRS estimator is configured to calculate a phase difference Φ betweentwo DMRS symbols.
 19. The apparatus of claim 18, wherein the DMRSestimator is configured to divide the phase difference Φ by a selectedtime interval T to determine the DMRS frequency offset estimate.